Carbon materials can be roughly divided into graphitic carbon, soft carbon and hard carbon according to the different carbon atom hybrid orbitals. Soft carbon and hard carbon are mainly used to describe carbon materials prepared by polymer pyrolysis. During the pyrolysis process, some carbon atoms are reconstituted into two-dimensional aromatic graphene sheets. If these graphene sheets are roughly parallel, it is easy at high temperatures For graphitization, this kind of carbon is called soft carbon; if these graphene sheets are randomly stacked and cross-linked by carbon atoms at the edges, and cannot be graphitized at high temperatures, this kind of carbon is called hard carbon. Generally speaking, graphitic carbon and soft carbon have high elasticity and are easy to deform, but the strength is low; due to the existence of a chaotic "house of cards" structure of hard carbon microscopically caused by a large amount of sp3-C, hard carbon materials are in mechanical strength and structure. It shows great advantages in terms of stability, but its intrinsic properties are relatively brittle and fragile. How to prepare hard carbon materials into superelastic blocks is currently a challenge.
Recently, inspired by the high strength and elasticity of spider webs in nature, a series of hard carbon aerogels with nanofiber network structures have been prepared by cleverly constructing a nanofiber network structure through a template method. This series of aerogels have the advantages of super elasticity, fatigue resistance and good stability.

Figure 1. Preparation of hard carbon aerogel. (a) Schematic diagram, showing a general synthesis method by using nanowires as templates; (b) Taking BCNF@RF as an example, mass synthesis of RF nanofiber hydrogel; (c) SEM image of hard carbon aerogel; (d) ) Shows the nanofiber network structure and fiber-fiber welding points.

Researchers used resorcinol-formaldehyde (RF) resin as a hard carbon source and used a variety of one-dimensional nanofibers as structural templates to prepare RF nanofiber aerogels. Super-elastic hard carbon aerogels can be obtained by high-temperature carbonization. glue. This kind of hard carbon aerogel has a fine microstructure, consisting of a large number of nanofibers and welding points between nanofibers (Figure 1). This method is simple and efficient, and easy to scale up production. By adjusting the addition of template and resin monomer, the diameter of nanofibers, the density of aerogels, and mechanical properties can be easily adjusted.
Unlike traditional hard and brittle hard carbon blocks, this hard carbon aerogel exhibits excellent elastic properties (Figure 2), mainly including: structural stability (after 50% compression, the microstructure can still be restored); High rebound speed (860 mm s-1), higher than many graphite carbon-based elastic materials; low energy loss coefficient (<0.16), the intermolecular forces existing in general graphite and soft carbon materials will cause adhesion And the friction force dissipates a lot of energy; fatigue resistance, after testing 104 cycles under 50% strain, the carbon aerogel only shows 2% plastic deformation and maintains 93% of the initial stress. The researchers also explored the application of this hard carbon aerogel in elastic conductors. After multiple compression cycles under 50% strain, the resistance is almost unchanged, showing stable mechanical-electrical properties, and can be used in harsh conditions. Under conditions (such as in liquid nitrogen) to maintain superelasticity and resistance stability.

Figure 2. Mechanical properties of carbon aerogels. (a) In-situ SEM of BCNF@C aerogel; (b) Comparison of energy loss coefficients of different materials; (c) Comparison of rebound speed of different materials; (d) Stress-strain of carbon aerogel under different cycles curve

Based on its excellent mechanical properties, this hard carbon aerogel is expected to be applied to stress sensors with high stability, large range (50 KPa), stretchable or bendable. In addition, this method can be extended to prepare other non-carbon-based composite nanofiber aerogels, providing a new way to transform rigid materials into elastic or flexible materials by designing the microstructure of nanofibers.